This invention relates generally to underwater communication devices and specifically to a simple, low cost, passive vocal communication snorkel. The art has long attempted to develop a passive, voice-powered communication device for enabling underwater communication. These devices were intended for use by scuba divers who carry tanks of compressed air for breathing. The stream of exhaust air bubbles, emitted from the swimmer's regulator, creates an ambient noise problem that renders intelligible underwater communication very difficult indeed.
A snorkeler, however, generally swims (or floats) just beneath the surface of the water and breathes through a breather tube that extends above the surface of the water. The breather tube is fitted to a mouthpiece that is held in the snorkeler's mouth. Since there is no stream of air bubbles being emitted, the ambient noise is much less of a problem. Yet, to date, there has been no successful passive underwater voice communication snorkel developed.
The characteristic impedance of water is 3600 times higher than that of air and therefore the communication of sound vibrations across an air water interface involves significant losses. The wavelength of sound in water is about 4.4 times greater than in air, hence a diaphragm size of 3 inch diameter, for example, is rather small to efficiently radiate voice frequencies in water. Consequently, such a diaphragm will exhibit substantial mass loading and reduced radiation impedance.
The base frequencies of the vocal chords, i.e. 100-200 Hertz, are normally suppressed when speaking in an air medium because the mouth area is too small to radiate them efficiently. These frequencies experience an increase of from 20-30 dB when speaking into a small enclosure such as a mask, a tube or the like. Additionally, a person's cheeks, throat and chest vibrate with low frequency sound, which is communicated to the water, forming a mass of unintelligible sound. This severely affects the intelligibility of the sound and is a major problem in developing a successful passive voice communication snorkel.
The voice communication snorkel of the invention solves many of the problems enunciated above. The preferred embodiment uses a pair of molded 0.008-0.014 inches thick, spherically shaped, polyester diaphragms that are mounted with an inward curve to decrease the volume of the cavity in the hollow snorkel body and to minimize the possibility of mechanical damage. The in-air resonance of the diaphragms are in the range of 3000 to 6000 Hertz. The mass loading of the water reduces their in-water resonance frequency to 1500-3000 Hertz. At the resonance frequency, the mass loading of the diaphragms is canceled by the stiffness of the diaphragm, and sound vibrations are coupled into the water. This supplies the higher voice frequencies which are necessary for intelligibility. While a two diaphragm snorkel is the preferred embodiment, a single diaphragm model will work, although with poorer intelligibility. Three or more diaphragms may be used with somewhat better results. In all multiple diaphragm cases, the individual diaphragms should be tuned to resonate at slightly different, overlapping frequencies to maximize the efficiency in covering said frequency range.
A major feature of the inventive snorkel is the mouthpiece containing a passageway that functions as a pair of "rubber lips", allowing air to be inhaled easily and yet providing for articulation of the exhaled air. This passageway is controlled by the lips of the snorkeler. A readily collapsible bubble is formed in the throat of the mouthpiece for engagement by the lower lip of a snorkeler. The bubble may be collapsed by minimal pressure from the snorkeler's lower lip to close the throat and help articulate plosive sounds.